There may be various objects to perform the lubricated hot rolling, such as to lower frictional force during rolling, and thereby reducing roll abrasion so that an energy-saving effect is attained; to enhance the quality of the produce surface; and so forth. For example, a technique to control (by supplying a lubricating oil) a surface damage of an HSS roll (e.g., high speed steel roll), which has become pervasive as a roll material for hot rolling. For example, Japanese Patent Application Laid-open Nos. Hei 05-306397, 08-188789, 6-79330 and 7-3279 describe that when a lubricating oil containing an high-basic alkaline-earth metal phenate, an high-basic alkaline-earth metal carboxylate, an high-basic alkaline-earth metal salicylate, an high-basic alkaline-earth metal sulfonate, or the like whose basicities are 40 mgKOH/g or greater, can be emulsified by applying a water injection supplying method (e.g., may be presently prevailing method) to perform hot rolling, a sticking-prevention effect may be enhanced, and a controlling effect of peeling of the oxide film (mill scale) on the surface of the HSS roll regarded as a cause of rough surface can be attained.
Furthermore, in the hot rolling, a plate is thicker than in cold rolling, and a feeding device of a material may not be provided, so that bit slippage and rolling slippage may likely occur. There are known techniques to solve this problem. For example, it is known to operate by reducing the amount supplied of the lubricating oil when the operation is carried out under such conditions that can obtain a lubricating effect within a range of not causing a slippage accident. A known method can be utilized in which the lubricated rolling is not performed before and after the top portion of a rolled material is bit into a rolling mill and the bottom portion thereof comes off the rolling mill, so that bit slippage is prevented. On the other hand, Japanese Patent Application Laid-open No. Hei 06-234989 describes a technique which uses a lubricant whose self-friction coefficient is high so that bit slippage and the like do not occur even if the lubricated rolling is performed to the top and bottom portions of the steel.
“Theory and Practice of Flat Rolling”, p. 218, The Iron and Steel Institute of Japan describes (as a lubrication supply method in hot rolling) a method in which a lubricating oil and vapor are blended and supplied in a spraying manner, other than the aforementioned method in which water and a lubricating oil are blended and supplied in a spraying manner in an emulsified form. On the other hand, Japanese Patent Application Laid-open Nos. Hei 11-279656, 11-279657 and 11-293345 describe (as a manufacturing method of hot rolling steel for deep drawing which is superior in uniformity of plate thickness) a technique outlined as that the amount supplied of a lubricating oil is 0.2 to 10 cm3 per 1 m2 of a surface area of the roll.
Further, Japanese Patent Application Laid-open No. 2003-94104 describes (as a lubricating oil supplying method not using water) a method to sprayedly supply to rolls the lubricating oil atomized or granulated to particulates using noncombustible gas, together with noncombustible gas. The supplying method can be known for: (i) providing a substantial friction-coefficient-reducing effect with a little amount supplied of the lubricating oil, (ii) reducing the friction force to the rolls, which reduces the roll abrasion, resulting in an effect of prolonging the life of the rolls; and (iii) providing a good resistance to disturbance since spraying of the noncombustible gas with the lubricating oil to the rolls allows the lubricating oil to reach the surface of the roll by blowing off the water film even if such water film formed by insufficient drying off of the roll cooling water exists on the surface of the roll.
When a lubricating oil being added with one kind or two or more kinds among the high-basic alkaline-earth metal phenate, high-basic alkaline-earth metal carboxylate, high-basic alkaline-earth metal salicylate, high-basic alkaline-earth metal sulfonate, and so forth, is used as an emulsified lubricant by being blended with water, the viscosity thereof may be higher than with conventional lubricating oils, so that nozzle clogging or pipe clogging is likely to occur. Accordingly, frequent cleaning of the pipe and nozzle may be required, which can lead to a deterioration of manufacturing efficiency. Further, in the case of the use of the emulsion-lubricated method, a dry-off wiper can be provided between a roll cooling water supplying section and a lubrication supplying section in order not to wet the lubrication supplying section with the roll cooling water. However, when the roll cooling water leaks into the lubrication supplying section through a clearance made by the rotation of the rolls and abrasion of the wiper, the lubricating oil supplied in the form of emulsified lubrication may become not easily adherable to the rolls. This can occur because the roll cooling water that leaks forms a water film over the surface of the roll, and further, the emulsion is blended with the cooling water so that its concentration (amount of the lubricating oil to water) is reduced, and the lubricating effect becomes difficult to be obtained.
It is known that approximately the same problem occurs with a supplying method in a steam-atomizing form. The concentration of the emulsified lubricant which is practically used is generally about 0.5 to 1.0 of lubricating oil to 100 of water in terms of ratio by weight. This range represents a range in which the slippage-accident prevention and the lubricating effect coexist. However, the concentration may be constantly changing by the dirt within the lubricant piping system, clogging of the nozzle, and further, a slight difference of viscosity of the lubricating oil by changing of temperature and humidity. A consequent problem that variation of lubricating effect occurs may also be recognized.
In order to solve these problems, if the method described in Japanese Patent Application Laid-open No. 2003-94104 is used, in which the lubricating oil is granulated into particulates and sprayed with the noncombustible gas to the rolls without using any water (hereinafter referred to as a gas atomizing method), a supply device with a very simple piping system can be formed. This may allow many of the above-described problems to be solved, and providing a firm resistance to external variable factors (such as, e.g., a seasonal change in viscosity or change in humidity). However, in this method, the lubricating oil itself can be directly supplied to the rolls near a steel material heated to approximately 800° C. to 1200° C., so that depending on the physicality and the usage environment of the lubricating oil, fire may break out in some instances. In particular, a lubricating oil being added with one kind or two or more kinds among the high-basic alkaline-earth metal phenate, high-basic alkaline-earth metal carboxylate, high-basic alkaline-earth salicylate, high-basic alkaline-earth metal sulfonate, and the like has a higher viscosity than lubricating oils not being added with them, so that when the lubricating oil is supplied in a manner described in Japanese Patent Application Laid-open No. 2003-94104, it is likely to scatter to or be deposited over the rolling mill facilities other than the rolls, causing a problem that the deposit flashes and sets a fire.